1. Field of the Invention
The invention relates to a method for a non-invasive examination of blood circulation in a living organism.
The measurement values of the blood circulation of a living organism such as a human being can be obtained on a continuous basis by means of invasive methods. In these methods a blood vessel is opened and a monitoring probe is introduced. In order to avoid the risk of infection, the use of non-invasive methods is desired for long-term monitoring.
Accordingly, it is known from Riva-Rocci and Korotkoff to automate the auscultation. In this monitoring method a sleeve, also called cuff, is placed at a given location on the examination subject (generally on the upper arm), where the pulsating flow of the arterial blood can be detected, e.g., by sound. The sleeve pressure is first set to a value that cuts off the flow of blood. The measurement of the diastolic and systolic pressure is made as the sleeve pressure falls. The cutting off of the flow of blood places a severe stress on the subject. The measurement can therefore only be performed as a single measurement and cannot be performed as a continuous monitoring of the circulatory system.
DE-OS 27 33 776 discloses a method for monitoring the pulse curve in which a sleeve is placed on the subject with a sensor for the sleeve pressure, which converts the measurement value into electrical values which are then evaluated oscillometrically. In this known method the sleeve pressure is put in place with a pressure that is less than the diastolic pressure of the subject, thus enabling a continuous monitoring of the pulse curve. The values of the diastolic, average, and systolic pressures can then be determined from the pulse curve. The following equation is valid for the sleeve pressure: EQU P.sub.M =P.sub.B1 -P.sub.TM (1)
wherein:
P.sub.M =sleeve pressure
P.sub.B1 =blood pressure
P.sub.Tm =transmural pressure.
Transmural pressure denotes the difference in pressure inside and outside a blood vessel. The expansion of the blood vessels due to transmural pressure is dependent not only on the age of the specific subject, but also on the temperature, muscle tone and nervous condition. These interrelationships are not fully understood. The pulse curve therefore does not offer a possibility for determining the actual curve of the blood pressure.
EP-PS 60 252 discloses a method in which the unknown function of the interrelationships of blood vessel expansion is eliminated from the blood pressure. In this method, light absorption is measured in the tissue of a finger of the subject and the sleeve pressure is automatically regulated in such a manner that the sleeve volume and thus also the blood vessel volume remain constant. This has the result, however, that despite the increase in blood pressure, the blood flow in the arteries located beneath the pressure of the sleeve is held constant, which places stress on the circulation of the subject and therefore cannot be used for long-term monitoring. Aside from this, the pressure in the sleeve must be regulated immediately, which necessitates a very expensive and complicated apparatus.
EP patents 152 848 and 188 894 disclose the use of a measurement with one or two sleeves at different pressures in a calibration phase, prior to a continuous monitoring of the blood circulation with one sleeve pressure below the diastolic blood pressure. Based on these measurements, parameters are thus established for determining the characteristics of the blood vessel walls.
Both methods share the disadvantage that the connection between blood pressure and blood vessel volume, as mentioned above, also depends, among other things, on the temperature, muscle tone and the nervous condition of the subject, and these factors can change very rapidly, especially with an ill subject. For example, a person in shock experiences a so-called vasorestriction, a constriction of the peripheral blood vessels, which has the effect that the brain is assured of an adequate supply of oxygen. All of these changes make it impossible with the known methods to perform a reliable, continuous monitoring of the circulatory system of a subject. In order to achieve an approximately precise representation of the actual circumstances, the calibration phase would have to be repeated often within short time frames, which would amount to a significant stress on the subject.
The above statements are valid even if the measurements are performed oscillometrically in accordance with the method in DE-OS 27 33 776, or in accordance with the methods contained in EP patents 24 772, 60 252 or 73 123 through the determination of light absorption.
DE-PS 21 52 688 discloses a method for the noninvasive measurement of the diastolic blood pressure, in which two sleeves are employed, the pressure chambers of which are connected with a common pressure discharge, operate with the same sleeve pressure, and are connected with each other by an intermediate indicator for pressure changes of flows. I this known method, these sleeves are placed at two points on the subject, one behind the other, on the same flow of arterial blood. The pulsing of the blood flow causes a pulsing expansion in the volume of the blood vessels surrounded by the sleeve, which results in a pulsing change in the sleeve pressure. When the instantaneous blood pressure falls below the sleeve pressure prevailing in the pressure chambers, there is a momentary closure of the blood vessels, or at least a significant reduction in the cross-sectional area thereof. This may result in different effects on the pulse-synchronous fluctuations in the blood vessel volume beneath the two sleeves, because behind the distal sleeve on the distal side lies a substantial volume of blood vessels and cell tissue which are blocked from the heart during the closure. The resulting differences in the instantaneous values of the pulsing change in sleeve pressure are displayed by the indicator. During the measurement, as the sleeve pressure falls, if the sleeve pressure becomes equal to or less than the diastolic pressure, then time-synchronous and equal quantity changes in blood vessel volume occur under both sleeves, and the indicator shows a minimum of its measurement value for the precise designation of the moment at which the sleeve pressure has reached the diastolic pressure.
In this known method it therefore depends on establishing the point at which the countervailing influences of two sleeves placed on a flow of blood cease.
DE-AS 27 51 004 discloses a device for vein closure plethysmography, in which a plurality of pressure sleeves are placed on one flow of blood. Here, too, one depends on the opposing influences of these sleeves so that the instantaneously measured sleeve pressures, which indicate blood pressure, are different in the two successively placed sleeves, as in the prior art described above in DE-PS 21 52 688.
Both in the known methods and in the method according to the invention, pressure sleeves are employed, having walls that are flexible but do not stretch under the pressures exerted during measurement.